The present invention relates to a PSK (phase shift keying) modulation circuit for digital communications systems.
In digital communications systems, particularly for satellite networks, precision PSK modulators are employed for interfacing data terminals and transmission media. Because of the inherent nonlinear characteristics of the diodes and transistors that comprise a PSK modulator of this type, their frequency response characteristics vary from one device to another. To ensure high precision PSK modulation performance, manual adjustments have been required to correct modulation phase errors and modulation amplitude errors whenever the carrier frequency is changed to transmit the data of a selected channel.
In a conventional satellite communications system such as Intelsat Business Service where a maximum of 3,200 data channels are available on a 140-MHz intermediate frequency band, a high precision local oscillator is used to supply the PSK modulator with a constant frequency carrier to be modulated with the baseband digital signal of a selected channel and the output of the modulator is converted in frequency by a mixer supplied with a variable frequency carrier from a channel synthesizer. Although this current PSK modulation technique can eliminate frequent adjustments and proved satisfactory for performance, the frequency conversion process introduces unwanted signals and requires an extra circuit for eliminating such unwanted signals. In addition, the use of a local oscillator adds to the complexity and the total cost of the system.